Within the context of the invention, the outlet exhaust line of a motor vehicle internal combustion engine comprises several elements for selective depollution of a pollutant, including first and second selective catalytic reduction systems. The second system is arranged downstream of the first system in the exhaust line, being separated by a spacing in relation to the first system. The first reduction system has an injector upstream of a reduction catalyst which injects an ammonia precursor reducing agent into the exhaust line for the reduction of nitrogen oxides. This may also be the case for the second reduction system.
Such a system is known as a selective catalytic reduction system, also known under the abbreviation SCR. An SCR system operates by injection, into the exhaust line, of a depollution agent referred to as SCR reducer, this agent advantageously but nonlimitingly being urea or a urea derivative, an ammonia precursor which is used to reduce nitrogen oxides or NOx.
In the following description, reference will be made either to the full name or the abbreviation SCR to denote everything associated with selective catalytic reduction. The same will apply to nitrogen oxides, which may be denoted by NOx, and to ammonia which may be denoted by NH3.
Such an SCR system may be integrated into a particle filter, as an alternative to an independent SCR system or as a supplement thereto. The particle filter is then impregnated with a catalyst to carry out selective catalytic reduction of NOR. When there are two SCR systems in the line, this applies equally to the first SCR system and to the second system.
As indicated above, the ammonia precursor reducing agent based on urea, the most commonly used of which is known under the name AdBlue®, reacts at high temperature to become ammonia or NH3, the NH3 reacting with nitrogen oxides or NOx, mainly in the form of a mixture of nitrogen monoxide, or NO, and of nitrogen dioxide, or NO2, in a ratio that varies especially depending on the engine operating conditions and on the temperature in the exhaust line. The depollution treatment of another pollutant or the maintenance of another depollution element, for example regeneration of a particle filter, may also influence the NO/NO2 ratio.
The decomposition of urea to give NH3 occurs according to the following equation:CO(NH2)2+H2O→2NH3+CO2 
This applies for mixtures based on urea as ammonia precursor reducing agent.
For the reduction of NOx, NH3 reacts in turn with the nitrogen oxides to form, by a reduction reaction, diatomic nitrogen and water. For example, with nitrogen monoxide, the reaction is written:4NO+4NH3+O2→4N2+6H2O
Another reaction with nitrogen monoxide and nitrogen dioxide is written:2NO+4NH3+2O2→4N2+6H2O
Other chemical reactions between NOx and NH3 are also possible.
For an exhaust line comprising first and second selective catalytic reduction systems arranged one after the other, the ammonia not used by the first SCR system for reducing NOx may arrive at the second SCR system. This may especially occur during a temperature increase in the exhaust line, for which ammonia is desorbed. Such a temperature increase in the exhaust line may occur during strong accelerations of the vehicle or during steady running at high engine speeds.
It is possible that the release of ammonia is greater than required to enable the reduction of NOx by the second SCR system. In this case, a surplus, referred to as a leak, of NH3 remains which is discharged into the environment upon leaving the exhaust line, when the temperature increase affects the second system. Since an emission of NH3 is a toxic emission, it is suitable to neutralize, or prevent the creation of, such an NH3 leak.
For this purpose, what is referred to as an active solution has thus been proposed, which provides control of NH3 storage at sufficiently low levels as a function of temperature.
This active NH3 storage control solution is based on a compromise between the depollution of NOx and the reduction of NH3 leaks, which entails increasing the emission of NOx, with optimal depollution of NOx therefore no longer being provided. Moreover, this solution is very complex and not particularly robust in command terms, especially when it is managed by an engine control unit.
There will therefore be an increase in the emission of NOx during accelerations of the vehicle when the NH3 storage control is too strict or the NH3 storage setpoint is too low. Conversely, there may still be an NH3 leak, that is to say a leak of NH3 not used for reduction, if the NH3 storage control is not quick enough or the NH3 setpoint is too high, relative to the increase in temperature, and the time required to consume the NH3 in the SCR catalyst of the second system.
Document US 2011/023463A1, incorporated by reference herein, is known, which relates to processes and systems for controlling a vehicle system having a first SCR region upstream of a second SCR region for emission control. In one example, when the temperature in the line at the second SCR region exceeds a maximum given temperature value, forced cooling is carried out of the second SCR region, which may be coupled to a cooling device to maintain a lower temperature than the first SCR region.
Other solutions, referred to as passive solutions, have also been proposed. One passive solution proposes an increase in the volume of the SCR catalysts. Another passive solution proposes using a catalyst for cleaning up waste ammonia, also referred to as “clean up catalyst” or “ammonia slip catalyst”, to remove the surplus NH3 not used for the selective catalytic reduction of the two consecutive SCR systems in the exhaust line. The clean up catalyst for the ammonia waste is located downstream of the two SCR systems in the exhaust line, advantageously in the downstream end portion of the exhaust line.
These solutions have the drawback of increasing the cost and bulk of the system by increasing the volume of the SCR catalyst, for the first passive solution, and by creating an additional catalyst, for the second solution.
Document US-A-2011/011060, incorporated by reference herein, describes an exhaust line having an SCR system and an active nitrogen oxide trap. During regenerations of the nitrogen oxide trap, which regenerations occur at regular intervals of time for emptying the trap of the NOx that it has adsorbed, since the emptying occurs under conditions of greater richness and therefore with a surplus of hydrocarbons, the trap releases ammonia. This ammonia is captured by the SCR system then used later for the catalytic reduction of NOx. This document proposes creating a Venturi effect between the NOx trap and the SCR system, which supplies sufficient air to keep the SCR system under poor richness or richness below 1.
Keeping the richness below 1 such as this makes it possible to protect the SCR catalyst from poisoning by hydrocarbons during regenerations and reduces losses of NH3 by oxidation. However, such a document does not discuss the problem of the association of two SCR systems placed one after the other in the exhaust line and gives no indication as to a reduction of leaks of NH3 that has not been used for reducing NOx and is therefore lost.